Semiconductor wafer manufacturing employs furnaces in which silicon wafers are heated in the presence of a flowing gas. These furnaces are typically referred to as diffusion furnaces, although a number of operations, such as oxidation, chemical vapor deposition, and annealing, can be carried out within them. Batch type furnaces hold, typically, 50 to 200 wafers. During operation, the wafers are placed into a process chamber and heated to the appropriate temperature, generally from 400.degree. C. to 1200.degree. C. depending on the process. A process gas is run through the chamber. During furnace operation, the temperature and gas flow are varied to achieve the desired reaction at the wafer. These variations must be accurately controlled to obtain uniform and consistent results, both within single wafers and from wafer to wafer across the entire wafer load.
A typical furnace employs a cylindrical quartz process chamber, referred to as a process tube. An appropriate semiconductor grade quartz is used for the chamber, because this material can withstand the high temperatures involved and can maintain chemical purity within the chamber. One end is open to allow the wafer load to be inserted and removed. The other end typically contains ports within a distribution manifold for the injection of the process gas. An outlet port is located at the opposite end of the tube for discharge of the gas. The process tube is held in an insulated electrical heater. The heater has an open end for insertion of the process tube. Within the process tube, the wafers are held in a boat that sits on an end cap. The end cap thermally isolates the wafers from the open end of the heater and seals against the open end of the process tube.
Thermal uniformity of the process chamber requires that the electrical heater be controlled in a number of zones along its length. One or more of the zones lie within the region nearest the gas inlet and end before or near the beginning of the wafer load. This region, called the non-product region, heats the end cap area and reduces heat loss through the furnace end.
The process rate must be uniform across the entire wafer load for uniform and consistent results. Hence the wafers and the reactant gas should be at a uniform temperature throughout the process chamber. Because the reactant gas comes from facilities that are much cooler than the furnace, entry of the gas into the furnace can adversely affect the uniformity of the wafer load, if the gas is not preheated to the furnace temperature prior to entering the process chamber.
In a typical prior art approach shown in FIG. 22, the gas passes through delivery lines that run contiguously from an inlet port at one end of the process tube, along the length of the process tube to the distribution manifold at the opposite end. In this approach it is intended that the furnace preheat the gas in the delivery lines before the gas enters the process chamber. With this type of configuration, however, some thermal non-uniformity and wafer imperfections still occur.